Current trends in gas turbine engines are moving towards so-called geared turbofan engines in which the fan is driven through a reduction gear train. The gear train allows the low pressure spool to be driven at higher rotational speeds which provides for a more efficient lighter engine core, whilst reducing the speed of the fan allows it to be a larger diameter thereby providing a higher bypass ratio. The reduction gear trains may be epicyclically configured where the fan is driven via the carrier of a planetary configuration, or a star configuration where the planet gears are fixed and the fan shaft is driven by the ring or star gear. The gear train may be a compound configuration as known in the art.
However, the introduction of the reduction gearing in large gas turbine engines leads to parasitic loads which result from an accumulation build tolerances and thermo-mechanical movement in service. The parasitic loads can result in significant deleterious loading and wear on the gears. For example, the drive shaft can suffer from yaw and pitch couples and radial loads which are placed on a sun gear and cause significant additional heat generation and wear in large gas turbine engines.
This is recognised partially in U.S. Pat. No. 7,591,754 which describes a coupling system for connecting the sun gear to a shaft within a planetary gear train. The sun gear coupling has an undulating flexible section joined to an inflexible spindle for accommodating misalignment between the sun gear and the drive shaft. The flexible section comprises a cylindrical ring having a diameter greater than the diameter of the spindle, and joined to the spindle by two longitudinally spaced apart diaphragms.
The present invention seeks to provide an improved drive for the sun gear of a gas turbine engine power gearbox.